Wire webmention.io receiver: account, rel="me", and deploy env

## Summary

Manual external account setup so the code in #144 has somewhere to
fetch from. webmention.io stores incoming webmentions; Bridgy
backfeeds Bluesky engagement as webmentions to it.

## Motivation

Sub-task split from #144 (Display webmentions on post pages). #144's
code can ship gated on `PUBLIC_WEBMENTION_IO_USERNAME`, but no
engagement appears until the webmention.io account exists, the value
is wired into the deploy, and the syndicated copies link back to
canonical URLs (Bridgy backfeed tracked separately in #201).

## Scope

- Verify #185 has shipped — the GitHub entry in `SOCIALS` must
render with `rel="me"` for IndieAuth to follow. ✅ live on deploy.
- Set the GitHub profile **Website** field to
`https://blog.alunduil.com` (GitHub renders the Website field as
`rel="me"` automatically, completing the reciprocal link). ✅
- Sign in to webmention.io via IndieAuth using
`https://blog.alunduil.com`. Username is `blog.alunduil.com`. ✅
- Wire `PUBLIC_WEBMENTION_IO_USERNAME=blog.alunduil.com` into the
deploy as committed IaC. The value is public (already emitted in
the page `rel="webmention"` link), so it goes in `deploy.yml`'s
build `env:` — not a GitHub Actions Variable or Secret. Lands with
PR #150. ✅ live in `pages.yml` and `weekly.yml`.
- Verify a test webmention round-trips: send via curl, confirm it
appears at `webmention.io/api/mentions.jf2?target=`, confirm next
build renders it (after #150 lands).

## Acceptance

- [x] #185 merged (`rel="me"` on GitHub link).
- [x] GitHub profile Website field set to `https://blog.alunduil.com`.
- [x] webmention.io account exists for `blog.alunduil.com`.
- [x] `PUBLIC_WEBMENTION_IO_USERNAME` wired into the deploy (PR #150).
- [ ] One end-to-end test mention rendered on a real post.

## Sub-issues

- #201 — Bridgy backfeed for Bluesky. Unblocked: #77 shipped.

## Related

- #514 — Threads backfeed, split out of #201. Bridgy has never
supported Threads, so it needs an upstream contribution and runs on
its own timeline; deliberately not a sub-issue, so this one is not
held open on a third-party repository.